1. Field of the Invention
This invention relates to pneumatic valve controls specifically to the reduction of heat tranfer from a hot valve body to a pneumatic valve actuator diaphragm.
2. Description of Prior Art
Many types of processes use pneumatic operated valves to control the flow of liquids and gases. The temperature of fluids controlled range from below 32 degrees f. as in brine service to 500 degrees f. or higher as in high pressure steam service. Many common processes using pneumatic control valves operate in the 45 degree f. to 450 degree f. range. The subject matter of my current invention relates to pneumatic actuated plug and seat valves operating in this range.
A modulating plug valve includes a body, with a plug and seat inside having a stem for raising and lowering the plug within the valve body. A passage inside the valve body allows fluid to flow through the valve when the plug is lifted off the valve seat using a attached valve stem and external spring assembly. A valve stem seal prevents fluid leakage from around the valve stem. A pneumatic actuator is used to close off the flow of fluid through the valve body. A pneumatic actuator includes a frame or housing to hold a flexible rubber diaphragm and a movable piston. The actuator mounts to the valve bonnet over the stem and spring assembly. Air pressure from a pneumatic source is applied to the top of the rubber diaphragm of the actuator which moves the piston down against the force of the spring and the force of the fluid pushing up on the plug and stem. As a higher air pressure is applied to the diaphragm the stem and attached plug moves down toward the seat until a full pneumatic pressure causes the plug to seal tight against the seat stopping fluid flow. Flow through the valve may be moudulated by controlling the air pressure to the actuator. Less air pressure=more flow. More air pressure=less flow.
It is often advantages to to mount the actuator close to the valve body to provide a low cost compact unit. At low fluid temperatures 45 degrees f. to 130 degrees f. these valve body and actuator combinations generally preform their function well, however their are several disavantages connected with their use when fluid and or ambiant air temperatures rise above 130 degrees f. Most aparent of these dissavantages is the close proxcimity of the valve body to the pneumatic actuator that contains the rubber diaphragm. A constant conduction of heat from the hot valve body to the rubber diaphragm through the actuator housing raises the temperature of the diaphragm. Radiant heat from the valve body raises the diaphragm temperature further. Pneumatic actuated plug valves are often used in areas that have high ambiant air temperatures such as a boiler room. These factors combine to greatly shorten the working life of the rubber diaphragm.
When diaphragm temperatures rise higher than 250 degrees f. sudden and unexpected diaphragm failure may result when the rubber diaphragm ruptures due to heat damage. This allows the valve stem to move to the full open position and the process or equipment being controlled by the valve to go out of control. This creates a dangerous condition to exist, cause damage or down time to equipment connected to the valve. Actuator Temperatures above 250 degrees f. require frequent diaphragm replacement to avoid costly equipment failure or process down time.
One way to lower actuator diaphragm temperatures is to replace the entire valve body and actuator with a special type of valve body and pneumatic actuator designed to operate at high fluid temperatures while protecting the valve actuator from excessive heat, however these valves have several significant draw backs.
One draw back is expense, special high temperature valves cost much more to manufacture than low temperature type valves, so much more as to cause some users to continue to use the lower priced pneumatic valves in high temperature applications creating hazardous conditions.
Another draw back is special high temperature valves are larger in size and in some cases would be difficult to install as expensive piping changes would be needed to accommodate its larger size. The expense of the new piping alone is often higher than the cost of a special high temperature valve.
This results in keeping the high maintanance pneumatic actuators in service as well as creating safety concerns.